The general structure of the electromagnetic clutch for use between an automobile engine and refrigerant compressor is disclosed in U.S. Pat. Nos. 3,044,594 and 3,082,933. The electromagnetic clutch disclosed in these patents, which is illustrated in FIG. 1 of the annexed drawings, comprises a pulley 1 rotatably supported on a tubular extension of a compressor housing through a bearing 2. The pulley is rotated by a belt coupled to the automobile engine. Pulley 1 is made from a magnetic material in which one or more concentric arcuate or annular slits 3 are formed to define a plurality of concentric annular magnetic pole pieces. Drive shaft 4 extends through a tubular extension in the compressor housing and a hub 5 is fixed to the terminal end of the drive shaft 4. An annular armature plate 6 is joined by a plurality of leaf springs 7 to hub 5 in such a fashion that armature plate 6 faces the annular concentric pole pieces with an axial air gap therebetween. A magnetic coil 8 is mounted on the tubular extension of the housing to generate magnetic flux to attract armature plate 6 to the magnetic pole pieces. Thus, when magnetic coil 8 is energized and the engine is driving pulley 1, drive shaft 4 rotates together with pulley 1. However, when magnetic coil 8 is not energized, even though pulley 1 is rotated by the engine, drive shaft 4 of the compressor is not driven.
In the above prior art construction, armature plate 6 is joined to hub 5 by a plurality of leaf springs 7 and, hub 5 is fixed to drive shaft 4. At the moment when armature plate 6 is energized, it impacts against the pulley 1, and an impact sound is caused because the two parts are formed of metal. This impact sound is very annoying to passengers within the automobile.
The impact force between armature plate 6 and pulley 1 occurs at the moment when the armature plate connects with the axial end surface of the pulley and the pulley begins to suddenly transmit rotational force. However, this impact force is not cushioned by the rigid connection of the leaf springs between the armature plate and the drive shaft. The sudden torque change is thus transmitted to the drive shaft of the compressor. The leaf springs are disposed at an angular offset to be drawn or pulled generally along their lengthwise dimension in the direction of the rotation. Therefore, if the clutch is rotated in a direction opposite to the normal direction of rotation, shearing stress is created in the leaf spring and the endurance of the clutch is therefore reduced.
Referring to another prior art design as shown in FIG. 2, a holder plate 9 is fixed, such as by rivets or other fastening means, on the opposite axial end surface of armature plate 6 than that which faces pulley 1. An inner end portion of holder plate 9 and outer end portion of hub 5 are formed with concentric annular axial flanges 91 and 51, and a rubber member 10 is disposed in the clearance between axial flanges 91 and 51. Therefore, armature plate 6 is connected to hub 5 through holder plate 9 and rubber member 10. The transmission of sudden torque changes to the drive shaft is thus eased by the rubber member. However, easing of the impact force is dependent on inducing of bending stress and/or shearing stress in the rubber member. Therefore, the rubber member fatigues by the repeated intermittent operation of the clutch and the rubber member becomes cracked or destroyed.